Field
The present disclosure relates to telecommunications apparatus and methods, and in particular to telecommunications apparatus and methods for use in wireless telecommunications systems in which terminal devices are configured to perform device-to-device communications.
Description of Related Art
The “background” description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description which may not otherwise qualify as prior art at the time of filing, are neither expressly or impliedly admitted as prior art against the present invention.
Mobile telecommunication systems, such as those based on the 3GPP defined UMTS and Long Term Evolution (LTE) architecture, are able to support more sophisticated services than simple voice and messaging services offered by previous generations of mobile telecommunication systems. For example, with the improved radio interface and enhanced data rates provided by LTE systems, a user is able to enjoy high data rate applications such as video streaming and video conferencing on mobile communications devices that would previously only have been available via a fixed line data connection.
The demand to deploy fourth generation networks is therefore strong and the coverage area of these networks, i.e. geographic locations where access to the networks is possible, is expected to increase rapidly. However, although the coverage and capacity of fourth generation networks is expected to significantly exceed those of previous generations of communications networks, there are still limitations on network capacity and the geographical areas that can be served by such networks. These limitations may, for example, be particularly relevant in situations in which networks are experiencing high load and high-data rate communications between communications devices, or when communications between communications devices are required but the communications devices may not be within reliable coverage of a network. In order to address these limitations there have been proposed approaches in which terminal devices (communications devices) within a wireless telecommunications system may be configured to communicate data directly with one another without some or all communications passing through an infrastructure equipment element, such as a base station. Such communications are commonly referred to as a device-to-device (D2D) communications. It is expected that D2D communications will be introduced in LTE release-12.
Thus, D2D communications allow communications devices that are in sufficiently close proximity to directly communicate with each other, both when within the coverage area of a network and when outside a network's coverage area (e.g. due to geographic restrictions on a network's extent or because the network has failed or is in effect unavailable to a terminal device because the network is overloaded). D2D communications can allow user data to be more efficiently communicated between communications devices by obviating the need for user data to be relayed by a network entity such as a base station, and also allows communications devices that are in sufficiently close proximity to communicate with one another when one or both devices may not be within the reliable coverage area of a network. The ability for communications devices to operate both inside and outside of coverage areas makes LTE systems that incorporate D2D capabilities well suited to applications such as public safety communications, for example. Public safety communications may benefit from a high degree of robustness whereby devices can continue to communicate with one another in congested networks and when outside a coverage area.
Fourth generation networks have therefore been proposed as a cost effective solution to public safety communications compared to dedicated systems such as TETRA (terrestrial trunked radio) which are currently used throughout the world.
The inventors have recognised one issue for consideration for D2D communications is how much radio resource should be reserved for supporting D2D communications in a wireless telecommunications network. A wireless telecommunications network generally supports communications over a wireless access interface comprising an array (grid) of time and frequency resources. In order to support D2D communications, at least in some modes of D2D operation, it is expected that a portion of a network's available resources will be reserved for D2D use. Radio resources which are reserved for supporting D2D communications in this way may sometimes be referred to herein as a D2D resource pool. The inventors have recognised in particular that the size of a D2D pool in a given implementation can be an important consideration for optimising network operation.
On the one hand, a relatively large D2D pool can support a correspondingly large amount of D2D communications, but with a corresponding large reduction in resources available for other communications in the network. This means if the resources comprising the D2D pool are not being used for D2D communications, the D2D pool will to some extent represent a waste of at least some resources in the network. On the other hand, a relatively small D2D pool will have a correspondingly smaller impact on the resources available for other communications, but gives rise to increased likelihood of congestion in respect of D2D communications. That is to say, there can be expected to be an increased chance of a device being unable to access resources to make D2D communications because the resources reserved for D2D communications (i.e. the D2D resource pool) are already allocated for use by other D2D devices. Public safety related activities are expected to represent one significant application of D2D communications, and for these applications it can be particularly important to avoid congestion.
The inventors have thus recognised a need for apparatus and methods for helping to make more efficient use of radio resources in networks supporting D2D communications while also helping to reduce the extent of D2D congestion in such systems.